Ranging by means of phase measurements on range tones transmitted around a closed circuit is a well-known method of determining distance between two points. Since transmission and reception on the same carrier frequency at the same time is not possible due to mutual interference, transmission of the range tones on path AB between stations A and B requires the use of a different RF carrier frequency than is used on the path BA.
Thus, such a distance measuring system requires two simultaneously occupied transmission channels and transmitters and receivers on two different frequencies. Such two frequency systems make inefficient use of the available ratio frequency spectrum.